In order to make high data rate interactive services such as video and internet access available to more residential and small business customers, high-speed data communications paths are required. Although fiber optic cable is the preferred transition media for such high data rate services, it is not readily available in existing communication networks and the expense of installing fiber optic cabling is prohibitive. Current telephone wiring connections, which consist of copper twisted-pair media, were not originally designed to support the data rates or bandwidth required for interactive services such as video on demand or even high speed internet connections. Asymmetric digital subscriber line (hereinafter referred to as “ADSL”) technology has been developed to increase the effective bandwidth of existing twisted-pair connections, allowing interactive services to be provided without requiring the installation of fiber optic cable. An ADSL modem can directly convert a stream of digital data into a large number of separate analog channels, operating at different frequencies and data rates to take advantage of all the available bandwidth of the telephone line. This discrete Multi-tone technology is designed to adapt to changes in the telephone line, due to temperature, or interference due to other calls and lines. To create multiple channels, ASL modems divide the available band-width of a telephone line.
By reusing the existing phone line, ADSL technology leads to further popularization of the Internet communication network. According to the ADSL standard G.DMT, approved by the International Telecommunications Union (ITU), a transfer channel is divided into many channels and data is transferred based on the state of the respective channels to widen a transfer bandwidth, with maximum data rates of 8 Mbps downstream to the subscriber and 1.5 Mbps upstream from the subscriber. G.Lite—Officially known as G.992.2, it is now a standard way to install ADSL service. G.Lite makes it possible to have Internet connections to home and business computers at up to 1.5 Mbps over regular phone lines. Using an extension to the ADSL standard, known as S=½, on 24-gauge wire, ADSL is capable of driving 10 mbps to nearly 10,000 feet. Thus, a very high-speed communication network may be implemented with ADSL technology. In an interactive ADSL communication system, a signal transmitted to the reception side along a loop line causes an echo. Since the echo results in a communication barrier, various efforts to cancel the echo have been made. A conventional echo canceller analyzes and predicts an echo generated when a loop is made, and cancels the echo at a received signal.
Conventional methods for canceling an echo are disclosed in U.S. Pat. No. 5,909,436, Japanese Patent Laid-Open Application No. 2000-049885, and Korea Patent Application No. 2001-09502. According to U.S. Pat. No. 5,909,436, a control register is disposed between a receive data buffer and a transfer data buffer. The two buffers have different sizes because of a difference between a data rate at a transfer side and a data rate at a reception side in a central office (CO) mode and a remote terminal (RT) mode. The control register is switched through software. However, since a filter coefficient is stored in a memory, a memory address control logic is needed.
According to the Japanese Patent Laid-Open Application No. 2000-049885, only a high SNR (signal-to-noise ratio) channel is selectively used and the other channels are mask-processed to lower a data rate. However, this patent supports a low sampling rate and thus cannot be practically applied to the current internet infrastructure.
According to the Korea Patent Application No. 2001-09502, an echo canceller includes a lattice-type filter unit which is used to remove the correlativity with respect to an input signal. An impulse response is presumed in an infinite impulse response (IIR) filter to shorten convergence time. Dozens of taps constitute one hardware. The echo canceller can have a high efficiency with respect to a voice signal having a high correlativity between signals, but is not suitable for current ADSL systems which must cancel the echo of 70-80 dB.
A conventional echo canceller (EC) circuit may use 512 FIFOs for the input data of 2.2M samples/s in the CO mode, and use 128 FIFOs for the input data of 552K samples/s in the RT mode.
Accordingly, there is a need for an echo canceller which is suitable to an ADSL communication system and shares the hardware in CO and RT modes to simplify its hardware structure.